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DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON. Director 



No. 380 

SPECTROPHOTOELECTRIC SENSITIVITY 
OF THALOFIDE 



BY 

W. W. COBLENTZ, Physicist 
Bureau of Standards 



JUNE 17, 1920 




PRICE, 5 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 
Washington, D. C. ' 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1920 



DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



Bureau of Standards 

S, W. STRATTON. Director 



No. 380 

SPECTROPHOTOELECTRIC SENSITIVITY 

OF THALOFIDE 



BY 

W. W. COBLENTZ, Physicist 

Bureau of Standards 



JUNE 17, 1920 




PRICE, 5 CENTS 

Sold only by the Superintendent of Documents, Govemxnent Printing Ofl&ce 

Washington, D. C. 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1920 



SPECTROPHOTOELECTRIC SENSITIVITY OF 
THALOFIDE 



By W. W. Coblentz 



1. INTRODUCTION 

Thalofide is a laboratory preparation of thallium-oxy-sulfid 
wtiich was discovered by T. W. Case^ to be photoelectrically 
sensitive. The material, after careful preparation, is fused upon 
a disk of quartz about 2 cm in diameter, and placed in an evacu- 
ated glass tube. The sensitivity is greatly increased in a vacuum, 
which preserves the life of the material by preventing oxidation. 

The material seems to imdergo a slow photochemical change 
(decrease in resistance), when exposed to blue and violet light, 
and hence it is mounted in an evacuated bulb of red glass. 

The first sample examined ^ was purchased from Mr. Case. It 
was in a red glass bulb, and was marked ''Type RL, No. 29." 
On communicating the results to Mr. Case, he generously of- 
fered to send another sample, which was mounted in an evacu- 
ated bulb of clear glass. This cell, No. 717, was several years 
old and had decreased somewhat in sensitivity. An empty red 
glass bulb was sent. By determining its spectral transmission 
it was possible to correct for the absorption of radiation by the 
red glass bulb which inclosed sample No. 29. 

The experimental procedure was the same as in previous work,^ 
except that a quartz prism was used for dispersing the radiation 
from the gas-filled tungsten lamp which was used for producing 
an equal energy spectrum. 

In order not to injure the cell by exposure to strong daylight, 
it was placed in a suitable light-tight, tubular moxmting (with a 
shutter), which could be slipped into the permanent ways which 
support the thermopile before the spectrometer slit.* Subse- 

^ Case Research Lab., Auburn, N. Y. See Phys. Rev., (2), 15, p. 289,11920; also U. S. patents Nos. i 301 227 
and 1 316 350 for a light-reactive resistance. 

2 Coblentz, Phys. Rev., 15, p. 139 ;i92o, Amer. Phys. Soc; Nov. 28, 1919. 

3 B. S. Bulletin, 15, p. 121; 1919. 

< B. S. Bulletin, 11, p. 132 (Fig. 3); 1914- 

253 



254 Scientific Papers of the Bureau of Standards [Voi, i6 

quently, for testing the effect of temperature, this mounting was 
modified by surrounding it with a small tin box, which could be 
filled with ice or water. The data presented herewith therefore 
represent two series of experiments in which the photoelectric 
cell had been remounted and, hence, the same parts of the photo- 
electric material were probably not examined each time. More- 
over, since the cells were mounted directly at the exit slit, and 
2 to 3 cm back of it, a wide portion of the photosensitive mate- 
rial was exposed to the radiation stimulus. 

In these tests the thalofide cell, which has a resistance of sev- 
eral megohms, was placed in series with a dry battery, a high- 
resistance, and a short-period d'Arsonval galvanometer. 

In view of its high sensitivity this device should be found use- 
ful in certain laboratory meastirements and in measiuing vari- 
able stars, etc. Its sensitivity is affected by change in temper- 
ature, hence for stellar radiation work it would have to be oper- 
ated in a thermostat. 

In common with all selective radiometers, it is quite imprac- 
tical to specify the radiation sensitivity of the thalofide cell in 
terms of heterochromatic radiation, such as the visual rays from 
a tungsten lamp. It is very interesting to find that this new 
substance lowers its electrical resistance by loo per cent on ex- 
posure to less than o.i m candle, using a ttmgsten filament. 
However, it should be remembered that this cell has its maxi- 
mum sensitivity at iju, where occurs the maximum emission of 
the tungsten lamp, so that the sensitivity test is principally for 
radiations at 0.9 to 1.2/i. On the other hand, selenium is quite 
insensitive to these radiations, but has its maximum sensitivity 
at 0.7JU, where the timgsten lamp is weak in radiation. Hence, 
no accurate comparison of the sensitivity of these two light- 
reactive substances is possible. However, speaking in general 
terms, this substance is very sensitive to the infra-red, just as 
the potassium hydride gas-ionic photoelectric cell is very sensi- 
tive to the violet rays 

By placing an absorption screen (say, of deep red glass and a 
water cell) in front of the cell, so as to obtain a fairly narrow spec- 
tral band of radiation at o.gix, and testing the sensitivity with a 
standard lamp (say, a vacuum tungsten lamp) , it may be possible 
to standardize the device for precise measurements. The cells 
examined were far more steady and reliable in action than selenium. 



Coblentz] 



Photoelectric Sensitivity 



255 



2. EXPERIMENTAL DATA 

The spectrophotoelectric sensitivity of thalofide is illustrated in 
Fig. I, in which curve B shows the behavior of sample No. 717, 
which was in the clear glass bulb. Curve A, Fig. i, shows the 
behavior of sample No. 29 (on 12 volts; dark current = 3 cm), 
after correcting for absorption of the red glass bulb. Further 
observations, uncorrected for absorption of the red glass bulb, are 
illustrated in Fig. 3. 

The two samples have practically the same spectrophotoelectric 
sensitivity, which consists of a complex, unsymmetrical band, with 
maxima in the region of 0.9 and iju. The sensitivity curve of 
thalofide is remarkable for the abruptness with which it terminates 



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Fig. I. — Spectro photoelectrical sensitivity curves of thalofide 

at 1 .2jLi, beyond which point this substance appears to be insensitive 
photoelectrically. 

For the intensities used, sample No. 29 seemed to fatigue a 
little in the course of an hour's work. 

Responsive-Time Curves. — ^The method of observation is to 
expose the sample to radiation and to read the galvanometer deflec- 
tion at short intervals. In Fig. 2 is shown the increase in the 
photoelectric current (galvanometer deflection) with time of 
exposure to radiation. The dotted parts of these curves illustrate 
the recovery of the cell after exposure to radiation. 

This substance is remarkable for its quickness of response, 
which is quite complete after a lapse of 1 5 seconds. On still longer 
exposure the galvanometer deflection increases slowly, and some- 
times irregularly, as indicated by the discontinuity in the curves in 
Fig. 2. Such an irregular increase in conductivity was noticed 
previously in molybdenite. 



256 



Scientific Papers of the Bureau of Standards [vu. i6 



While the time (two minutes) to attain a maximum response was 
shorter than that of molybdenite, this substance behaves somewhat 
like it in requiring twice as long (four minutes) for complete 
recovery. For small deflections, an expostire of one minute, with 
two minutes for recovery, was sufficient. 

Fig. 2, ctuve A, illustrates the response of thalofide cell No. 717, 
and curves B and C illustrate the response of cell No. 29. 

Ei^FECT OF Temperature. — It was of interest to determine the 
effect of temperature upon the spectrophotoelectric sensitivity. 
For this purpose the cell was placed in a bath of ice, as already 
described. Since the thalofide tube was not directly in contact 
with the ice (or hot water), the photosensitive material was 




C 1 t J — J ' --^ ^^M IW, 

Fig. 2. — Variation of photoelectrical current with time of exposure to radiation 

probably not cooled to 0° C, though for convenience that tem- 
perature is used. 

In Fig. 3 the effect of temperature upon thalofide cell No. 29 is 
illustrated. These curves are not corrected for absorption of radia- 
tion by the red glass bulb. Furthermore, they are for the cell 
remoimted in the holder, as already mentioned. The applied 
potential (12 volts) was kept constant, and at room temperature 
(18° C) the dark current was 1.6 cm, while at 0° C the dark 
current was only 7 to 8 mm. 

Nevertheless, keeping the radiation intensity constant, at 0° C 
the photoelectric sensitivity (ciurent) is about three times as 



Coblenfz] 



Photoelectric Sensitivity 



257 



large as that obtained at 18° C. Since at a given temperature 
the photoelectric current is proportional to the dark cinrent, as 
observed on actual test, if we had raised the voltage sufficiently 
to produce a dark current of 1.6 cm, the photoelectric current 
at 0° C, for radiation of wave length X=iju, would have been 
about six times that obtained at 18° C. 

In Fig. 4 is shown the effect of temperature upon thalofide cell 
No. 717. At room temperature (16° C) the dark current was 
2.6 cm on about 30 volts. At 0° C the applied potential was 
about 50 volts, producing a dark current of 2.2 cm. At 35° C 




Fig. 3. — Effect of temperature upon the spectrophotoelectrical sensitivity of thalofide cell 

No. 2Q (red glass bulb) 



the dark current through the cell, on 20 volts, was 5.9 cm. Never- 
theless, the maximum (photoelectric) galvanometer deflection was 
only 2.1 cm. If the dark current had been reduced to 2.6 cm, 
the maximum deflection, at 35° C, would have been only about 
0.9 cm, as compared with 5.8 cm observed at 0° C. 

The curves illustrated in Fig. 4 show that, in common with 
nearly all photoelectric substances examined, the sensitivity 
increases the most rapidly on the short wave-length side of the 
maximum, and the maximum of the sensitivity curve shifts 
toward the short wave lengths, with decrease in temperature. 



258 Scientific Papers of the Bureau of Standards 

3. SUMMARY 



[Vol. i6i 



Experimental data are given on the spectrophotoelectric 
sensitivity of Case's preparation of thallium-oxy-sulfid, thalofide, 
when exposed to thermal radiation of wave lengths extending 
from 0.58 to 3iu. 

It was found that this substance has a wide, unsymmetrical, 
complex band of photoelectric sensitivity, which extends from 
the visible into the infra-red, where it terminates abruptly at 1.211, 




Fig. 



-Effect of temperature upon the spectro photoelectrical sensitivity of thalofide 
cell No. 7/7 (clear glass bulb) 



There is a maximum at ijn and probably a smaller maximum at 
0.85M. 

The effect of temperature, from o to 35° C, was investigated. 
It was found that, in common with nearly all photoelectric sub- 
stances thus far examined, the sensitivity increases the most 
rapidly on the short wave-length side of the maximum, and the 
maximum of the sensitivity curve shifts toward the short wave 
lengths, v/ith decrease in temperature. 

Washington, January 3, 1920..